The present invention relates to an image forming technology for forming multicolor images using area gradation (by which gradation is set by the sizes of dots in pixels) by thermal transfer and, more particularly, to an image forming technology which uses a method (to be referred to as a dot-on-dot method hereinafter) which obtains a predetermined color by stacking dots having different colors in substantially the same spot.
A printing method is practically most widely used among other methods of writing images on a medium on the basis of image information. Other technically possible examples of the methods are a thermal transfer method to be described in the present invention, electrophotography method, ink-jet method, thermal destruction method, and various transfer recording methods using photopolymerization recording materials.
Unfortunately, any of these methods has some problems, e.g., difficulty in forming an image directly on a final recording medium (a final product) to which the image is to be given, low mass-productivity, and high cost. In cases like these, an image is formed on an intermediate transfer medium and then transferred from this intermediate transfer medium onto a final product.
When an image forming method is a thermal transfer method using, e.g., a sublimating dye, the operation is performed following a procedure explained below as is well known. First, a thermal transfer ribbon coated with a sublimating dye in a thermally transferable form and a target body as a final recording medium are overlapped on a substrate film. Subsequently, the thermal transfer ribbon is selectively heated by using a thermal head or the like on the basis of image data, thereby recording a desired image on the target body by transfer.
When the faces of different persons are to be separately recorded on different target bodies, for example, the above means can easily record a number of different images as color images having rich gradation on target bodies. This is the advantage which the printing method does not have. That is, if the printing method is used to record the faces of different persons, enormous cost, labor, and time are generally required, resulting in very poor economy.
On the other hand, materials which can be dyed by sublimating materials are limited. That is, it is possible to use only target bodies made of limited materials such as polyester, acrylic resin, and vinyl chloride resin. Hence, when thermal transfer recording using a sublimating dye is to be performed although a material other than these materials is used as a target body, some improvements are necessary as disclosed in, e.g., Jpn. Pat. Appln. KOKAI Publication No. 63-81093. In this reference, an image writing unit using a transfer ribbon of a sublimating dye and a thermal head first writes an image on a film-like intermediate transfer medium having an adhesive layer. Subsequently, a transfer unit transfers the image on this intermediate transfer medium together with the adhesive layer onto a target body by heat and pressure.
The above method is an example using a sublimating dye. In the following description, however, methods which use coloring materials other than a sublimating dye and by which an image is once formed on an intermediate transfer medium and then transferred, together with the layer in which it is formed, from this intermediate transfer medium onto a target body will be generally referred to as indirect transfer methods.
In some cases, however, images cannot be directly formed on target bodies, or enormous cost and time are required if images are to be actually formed. This happens due to various reasons when, e.g., a target body as a final product (a recording medium) has a nonuniform thickness, has a rough surface (a typical example is a contactless IC card), or is a semi-completed product such as a booklet (a typical example is a passport). In such cases, images can be formed only by indirect transfer methods in practice.
If electrophotography is used as a method of writing an image on an intermediate transfer medium on the basis of image information and if the image is a full-color image, an electrophotographic process must be repeated three times (for three colors) or four times (for four colors). The electrophotographic process of each color includes charging of a photosensitive body, formation of a latent image on the charged photosensitive body by exposure, development of a toner image corresponding to the latent image on the photosensitive body, transfer of the image to a transfer member such as a transfer drum for temporarily storing the toner image of the corresponding color, erasure of unnecessary charged portions on the photosensitive body, cleaning of the photosensitive body, and the like. In this case, therefore, the process is time-consuming and, in addition to that, it is necessary to prevent unstable image formation resulting from the use of static electricity which is very unstable.
Furthermore, since the sizes of dots of toner images for forming an image cannot be largely changed, the image is basically a binary image. Accordingly, a density change of an image cannot be expressed without using the method of pseudo area gradation using a dither matrix of Bayer type or Fatton type (including screw type). As a consequence, an image itself is coarse.
When the ink-jet method is used, on the other hand, an image is formed on an intermediate transfer medium by using a liquid ink, so the image must be dried. This also poses a problem of nozzle clogging. Additionally, since the sizes of dots cannot be largely changed even in the ink-jet scheme, the method of pseudo area gradation such as a dither matrix or an error diffusion method is used. This often decreases the resolution of an image.
Note that the thermal destruction method cannot form a full-color image at present.
For the reasons described above, image formation by the thermal transfer method using a sublimating dye is simple and inexpensive and can achieve high image quality and high resolution. Accordingly, this method is superior as an image forming method to other indirect transfer methods.
Unfortunately, this thermal transfer method using a sublimating dye has a large drawback: a sublimating dye itself is a coloring material very inferior in so-called resistances, e.g., heat resistance, light resistance, and solvent resistance. Hence, when a sublimating dye is used, the durability of an image on a target body as a final product significantly lowers. For example, even when a target body is an IC card having a heat resistance of about 120xc2x0 C., a decrease in image density due to a phenomenon such as thermal decomposition or resublimation of a sublimating dye occurs at about 80xc2x0 C. That is, no sublimating dye can have a heat resistance exceeding a heat resistance of 120xc2x0 C. of a target body.
Also, when paper such as a passport is used as a target body, an image transferred onto the paper surface xe2x80x9coozes outxe2x80x9d from the back side owing to the ambient of a solvent such as paradichlorobenzene or naphthalene often used as a mothproofing agent. Additionally, a sublimating dye resublimates from the paper fibers at high temperatures, and this lowers the image density.
Furthermore, since the sublimating printing method is in widespread use in the world, if this method is used for a security purpose of, e.g., a passport, the passport is readily forged or altered. In addition, this forgery or alteration cannot be easily found.
To solve these problems unique to a sublimating dye and achieve simplicity, low cost, high image quality, and high resolution of the thermal transfer method, a melt-transfer printing method using area gradation is very effective. This method obtains gradation by changing the sizes of dots to be transferred in accordance with the amount of heat generated by a thermal head used in thermal transfer. That is, area gradation is possible by changing a region in which an ink-ribbon ink is softened or melted, in accordance with the controlled heat amount from the thermal head.
In this method, an ink ribbon is formed by previously applying an ink onto a substrate film such as polyethyleneterephthalate (to be abbreviated as PET hereinafter) or polyethylenenaphthalate (to be abbreviated as PEN hereinafter) by a printing method or the like. An ink is formed by appropriately internally adding an organic dye or a coloring material such as an organic or inorganic pigment to a binder resin, e.g., polymethylmethacrylate, polybutyral, or a vinyl chloride-vinyl acetate copolymer, and internally adding a wax component, filler, and the like if necessary.
Since in this method a dye other than a sublimating dye or a pigment can be used as a coloring material, the durability such as the heat resistance, solvent resistance, and light resistance can be greatly improved. Accordingly, the method has high requirement conformity in the fields of, e.g., a passport, visa, and auto-driving license, requiring high durability.
Also, the melt-transfer method using area gradation is very sensitive to the roughness of a recording medium; images cannot be directly transferred or formed if a recording medium has even a slight roughness. This makes the melt-transfer method suitable to the indirect transfer method. In other words, it is nearly impossible to obtain high-quality images by the melt-transfer method using area gradation unless the indirect transfer method is used.
Methods (dot mapping) of arranging dots of different colors when the above-mentioned area gradation is to be formed by a color image, i.e., multicolor inks, are roughly divided into two methods.
One is a screen method widely used in, e.g., an offset printing method. The other is a method of arranging dots of different colors in substantially the same spot, i.e., a dot-on-dot method.
First, the screen method will be described below.
When dot images (point images) of two or more colors are to be mapped, dots formed by a thermal head form a substantially regular dot array. For example, when a thermal head having a resolution of 300 dpi (the dpi is a unit indicating the number of dots per inch) in the main scan direction is used and dots are mapped by the same resolution of 300 dpi in the sub-scan direction, these dots form a mass of lattices at intervals of approximately 85 xcexcm. Note that in this specification, the main scan direction is the longitudinal direction in which heat-generating portions of a thermal head are arrayed, and the sub-scan direction is perpendicular to this main scan direction.
When such regular dot masses are recorded on a recording medium by transfer (in the present invention, this corresponds to image formation on an intermediate transfer medium), slight differences to some extent are unavoidably produced between the mapped positions of different colors owing to a positional deviation (caused in many cases by, e.g., velocity variations in the sub-scan direction or holding slip of the recording medium) in the sub-scan direction. If a slight positional deviation is present when different colors are overlapped although each single color is regular, this deviation component induces a beat phenomenon with the mapping of each color, resulting in unfavorable xe2x80x9cmoirexe2x80x9d on the recorded image.
To shift the mapping position of each color in advance, therefore, the angle of lattice-like mapping is changed (the screen angle is changed), or the resolution of the color is changed (e.g., one pixel is formed by two dots). In either case, the appearance of moire is prevented by using a method of performing dot mapping so as to intentionally prevent regular overlapping of dots having different colors, i.e., by using a screen method.
When this screen method is used, however, the apparent resolution (the gradation resolution) lowers (to 75 to 150 dpi for a 300-dpi thermal head). In addition, individual colors are apparently arranged at random, and this sometimes makes images look rough. Furthermore, each color image must be changed into a screen image. This imposes a large load on an internal control CPU of a printer or on a CPU of a host computer or the like which sends image data to a printer, thus finally delaying the time of issue greatly.
Also, when an image is formed on a target body for a security purpose such as a passport by using this mapping method, the image looks analogous to those formed by offset printing and gravure printing. This makes the characteristics of the printing method difficult to use to achieve the effect of suppressing illegal use such as alteration or forgery.
On the other hand, the dot-on-dot method is a method of mapping dots having different colors in substantially the same position with high accuracy. Therefore, problems such as moire and apparent color tone shift caused by color shift do not occur unless the positions of these colors deviate from each other. Also, images can be formed with the maximum resolution of a thermal head. In addition, since image mapping is not basically changed, no load is imposed on a CPU. As a consequence, the speed of issue can be increased.
Unfortunately, this dot-on-dot method has scarcely been put into practical use because no technique for accurately overlapping different colors has been established.
The present invention has been made in consideration of the above-mentioned problems of the conventional techniques, and has as its object (the first object) to provide an image forming apparatus and method which, when an image is to be recorded on an intermediate transfer medium by transfer, can achieve area gradation by using substantially truly circular dots in a dot-on-dot manner by improving a driving system of a holding member, such as a platen roller, for holding the intermediate transfer medium, and improving a thermal head as a writing device.
It is another object (the second object) of the present invention to provide an image-applied article formable by the above image forming apparatus or method and highly effective to prevent illegal acts such as alteration and forgery or highly effective to facilitate finding such illegal acts.
The first aspect of the present invention is an image forming apparatus which uses a thermal transfer ribbon having a plurality of ink layers of different colors containing a coloring material selected from the group consisting of a pigment and dye, and a long, film-like intermediate transfer medium capable of transferring the ink layers from the thermal transfer ribbon, comprising
a platen for holding the intermediate transfer medium when the ink layers are transferred from the thermal transfer ribbon to the intermediate transfer medium,
a driving mechanism which comprises a driving source and transmission members and drives the platen, the transmission members being interposed between the driving source and the platen to mesh with each other and having a speed reducing ratio which is an integer multiple,
a thermal head which has a substantially regular polygonal or substantially circular heat-generating portion and selectively heats the thermal transfer ribbon while the intermediate transfer medium and the thermal transfer ribbon are overlapped on the platen, thereby selectively transferring the ink layers onto the intermediate transfer medium,
control means for forming a record image containing an area gradation image on the intermediate transfer medium by driving the thermal head, on the basis of image information, in collaboration with driving of the platen by the driving mechanism, the area gradation image being made up of sets of dots having different colors formed by the ink layers and having a color set by stacking the dots having different colors in substantially the same spot, and heating and pressing means for overlapping the intermediate transfer medium on which the record image is formed and a target body and applying heat and pressure to the intermediate transfer medium and the target body, thereby transferring the record image from the intermediate transfer medium onto the target body.
The second aspect of the present invention is an image forming apparatus according to the first aspect, wherein the intermediate transfer medium comprises an image-receiving layer, and the record image is formed on the image-receiving layer and transferred together with the image-receiving layer onto the target body.
The third aspect of the present invention is an image forming apparatus according to the first aspect, further comprising punching means for punching the intermediate transfer medium along the contour of the target body and transferring the record image together with the punched portion of the intermediate transfer medium onto the target body.
The fourth aspect of the present invention is an image forming apparatus according to any one of the first to third aspects, wherein the record image further contains a binary image.
The fifth aspect of the present invention is an image forming apparatus according to any one of the first to third aspects, wherein the driving source is a stepping motor driven by the number of steps by which a speed reducing ratio is an integer multiple with respect to the transmission members.
The sixth aspect of the present invention is an image forming method which uses a thermal transfer ribbon having a plurality of ink layers of different colors containing a coloring material selected from the group consisting of a pigment and dye, and a long, film-like intermediate transfer medium capable of transferring the ink layers from the thermal transfer ribbon, comprising
an image forming step of forming a record image containing an area gradation image on the intermediate transfer medium on the basis of image information by repeating an operation of selectively heating the thermal transfer ribbon by a thermal head while the intermediate transfer medium and the thermal transfer ribbon are overlapped on a platen, the area gradation image being made up of sets of dots having different colors formed by the ink layers and having a color set by stacking the dots having different colors in substantially the same spot, and a driving mechanism of the platen comprising a driving source and transmission members interposed between the driving source and the platen to mesh with each other and having a speed reducing ratio which is an integer multiple, and
a heating and pressing step of overlapping the intermediate transfer medium on which the record image is formed and a target body and applying heat and pressure to the intermediate transfer medium and the target body, thereby transferring the record image from the intermediate transfer medium onto the target body.
The seventh aspect of the present invention is an image forming method according to the sixth aspect, wherein the intermediate transfer medium comprises an image-receiving layer, and the record image is formed on the image-receiving layer and transferred together with the image-receiving layer onto the target body.
The eighth aspect of the present invention is an image forming method according to the sixth aspect, further comprising the punching step of punching the intermediate transfer medium along the contour of the target body and transferring the record image together with the punched portion of the intermediate transfer medium onto the target body.
The ninth aspect of the present invention is an image forming method according to any one of the sixth to eighth aspects, wherein the record image further contains a binary image.
The 10th aspect of the present invention is an image forming method according to the ninth aspect, wherein the image forming step comprises a step of forming, as the binary image, micro characters made up of elements selected from the group consisting of characters, numbers, symbols, seals, and patterns, by using sets of the dots.
The 11th aspect of the present invention is an image forming method according to the ninth aspect, wherein the image forming step comprises a step of forming, as the binary image, a pattern for generating moire when the record image is read by a scanner, by using sets of the dots.
The 12th aspect of the present invention is an image-applied article comprising a substrate, a record image, and a transparent resin layer formed on the substrate to cover the record image such that the record image is visible, wherein the record image contains an area gradation image and binary image, the area gradation image is made up of sets of dots having different colors formed by ink layers and has a color set by stacking the dots having different colors in substantially the same spot, the binary image comprises micro characters formed by using sets of the dots and made up of elements selected from the group consisting of characters, numbers, symbols, seals, and patterns.
The 13th aspect of the present invention is an image-applied article according to the 12th aspect, wherein the micro characters represent personal information pertaining to a main part of the record image.
The 14th aspect of the present invention is an image-applied article comprising a substrate, a record image, and a transparent resin layer formed on the substrate to cover the record image such that the record image is visible, wherein the record image contains an area gradation image and binary image, the area gradation image is made up of sets of dots having different colors formed by ink layers and has a color set by stacking the dots having different colors in substantially the same spot, the binary image comprises a pattern formed by using sets of the dots to generate moire when the record image is read by a scanner.
The 15th aspect of the present invention is an image-applied article according to the 14th aspect, wherein the pattern for generating moire is formed such that thin lines extend in a plurality of different oblique directions by dots formed at a high-resolution pitch.
In the present invention as described previously, dots having different colors are formed in substantially the same spot as one requirement for forming an image by area gradation. The meaning of xe2x80x9csubstantially the same spotxe2x80x9d includes very slight positional deviations between stacked dots having different colors. That is, xe2x80x9csubstantially the same spotxe2x80x9d mentioned in the present invention includes a case in which, of stacked dots different in color, the distance between the centers of dots of colors most deviated from each other is within approximately ⅓ the dot formation pitch corresponding to the resolution. In the present invention, to obtain a high-quality, high-area gradation image, it is important to stack dots with very high positional accuracy such that the center-to-center distance is preferably within xc2xc the dot pitch.
Also, a xe2x80x9csubstantially regular polygonal shapexe2x80x9d or a xe2x80x9csubstantially circular shapexe2x80x9d of the heat-generating portion mentioned in the present invention naturally includes a true regular polygon (including a true square) or a true circle. However, this xe2x80x9csubstantially regular polygonalxe2x80x9d or xe2x80x9csubstantially circularxe2x80x9d shape is not necessarily restricted to a true regular polygon or true circle. The whole heat-generating portion corresponding to one dot need only have a shape macroscopically similar to a regular polygon or circle.
That is, the corners of a polygon can be chamfered or rounded with a small radius, or its contour need not be partially or entirely composed of straight lines or curved lines. Simple examples are: (1) an octagon (not a regular octagon) having eight corners but assuming a shape similar to a square whose four corners are slightly chamfered; (2) a pentagon (not a regular pentagon) substantially close to a square, i.e., four interior angles are close to 90xc2x0 but the remaining one interior angle is extraordinarily large (around 180xc2x0); and (3) a shape formed by rounding the four corners of a square, which is not a polygon (or a regular polygon) because it has no corners. Any of these shapes corresponds to a xe2x80x9csubstantially regular polygonal shapexe2x80x9d mentioned in the present invention. Also, a xe2x80x9csubstantially circular shapexe2x80x9d can be an ellipse or a more or less distorted circle in a strict sense.
The number of corners of a regular polygon is not limited to a specific once, i.e., a regular polygon can have any number of corners as long as the polygon can be manufactured in practice. When the number of corners increases, the shape ultimately becomes close to a true circle. Also, when the number of corners of a regular polygon is small, favorable results meeting the objects of the present invention are readily obtained if the number is an even number rather than an odd number. When the number of corners is large, no big difference is produced regardless of whether the number is an odd number or even number.
As a macroscopic dimensional ratio of the shape of the heat-generating portion, the ratio of the width of a widest portion of the shape to the width in a direction perpendicular to the direction of the widest portion is preferably as close to 10:10 as possible, regardless of whether the shape is a xe2x80x9csubstantially regular polygonalxe2x80x9d or xe2x80x9csubstantially circularxe2x80x9d shape. However, even if this ratio more or less changes, there is a range within which well favored results are obtained in practice. Although this range cannot necessarily be defined, a rough standard is about 10:7 to 7:10.
In practice, a square, a rectangle close to a square, or a shape substantially close to these shapes is preferred because of the ease of design and manufacture and the power of influence with which favorable results meeting the objects of the present invention are obtained.
Note that one heat-generating portion usually forms one dot on a target body. However, when a heat-generating portion for forming one dot on a target body is composed of a plurality of small heat-generating portions, the whole of these small heat-generating portions for forming one dot need only macroscopically have a xe2x80x9csubstantially regular polygonalxe2x80x9d or xe2x80x9csubstantially circularxe2x80x9d shape.
When the heat-generating portion of the thermal head has a substantially square or circular shape, formed dots are also circular dots, and this facilitates area gradation. Note that when a thermal transfer ribbon in which an ink layer is formed on a substrate film and the thickness of this ink layer is 1 xcexcm or less, the ink layer can be easily cut, so area gradation can be readily performed.
The platen is driven by synchronous drive transmitting means, such as timing belts or gears, which produce no slip, and each driving speed reducing ratio is set to be an integer multiple. Accordingly, the ripple periods of the power transmission torque ripples of individual reduction gears are equal to each other. Therefore, images can easily be formed by beautifully overlapping dot images of different colors.
A representative example of a particularly suitable thermal transfer ribbon is the one disclosed in Jpn. Pat. Appln. KOKAI Publication No. 7-117359 (U.S. Pat. No. 5,726,698) (in this reference, the thermal transfer ribbon is represented as a xe2x80x9cthermal transfer recording materialxe2x80x9d). By the use of this thermal transfer ribbon, images having undergone good area gradation can be formed by a heat-bonding thin film peeling method (represented in the above reference).
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.